Cleansing of surfaces by the blast of an airstream carrying particles of dry ice. Apparatus providing a selectible capability of either metering preformed dry ice particles while substantially maintaining the size and distribution integrity of said particles or the capability to granulate a block of solid preformed dry ice without modifying the apparatus itself.
The use of dry ice (solid CO2) particles in a pressurized airstream to clean surfaces is known art. In this process, the dry ice particles are conveyed through a hose to a nozzle from which the air and the entrained particles are discharged onto the surface that is to be cleaned. The fact that dry ice sublimes directly to a gas makes these particles particularly suited for blasting onto many types of surfaces with the advantage that they leave no residue and are less likely to modify critical surface characteristics.
While in the solid condition, particles of dry ice have useful structural properties for blast cleaning applications. They are able to dislodge and sweep away surface contaminants such as paint on heavy solid surfaces, and preservative coatings on structures as fragile as small electrical coils. Obviously a particle large enough and structurally integral enough to remove a baked-on enamel from a heavy metal structure cannot be used to cleanse the surface of a delicate wire coil. It would destroy the coil. Yet the same materialxe2x80x94dry icexe2x80x94can be used successfully for both purposes.
The difference resides in the size and structure of the particles. For heavy blasting, where it is advantageous that each individual particle impact generates significantly higher kinetic energy, it is common to use preformed dry ice particles in pellet form of a predetermined size and distribution. Such pellets are separately prepared by equipment specifically designed to produce them from a bulk supply of liquid carbon dioxide. They are placed as pellets in a storage bin or hopper in the dry ice blasting apparatus and are metered from it into the airstream and conveyed to an accelerator either through eduction or through a pressurized airstream requiring the use of an airlock. An example is found in U.S. Pat. No. 4,038,789.
However, this same equipment cannot dispense the smaller granular particles that are preferred in many applications. These dry ice granules cannot optimally be stored for subsequent metering because their highly hygroscopic nature coupled with a high surface to mass ratio that produces a strong tendency to clump when stored for even very short time period. In addition, even when very small quantities of dry ice granules are stored, the weight of the granules in the upper portion is sufficient to combine and compress the particles below them into undesirable larger sizes of dry ice.
This challenge was resolved by equipment as presented in U.S. Pat. No. 5,520,572. Such particles are customarily extracted by scraping or shaving preformed dry ice (usually in block form but it could also be in nugget or pellet form) and then immediately feeding the dry ice granules into the airstream at a selectable rate with substantially no storage. However, the conventional granulated dry ice blasting apparatus which existed prior to this invention, when used to create and then inject such particles into an airstream, could not also instead satisfactorily meter the larger preformed dry ice pellets. If this was attempted without significantly modifying the granulator mechanism it would result in damaging pellet integrity and thus changing the cleaning characteristics of the dry ice particles.
To convert a machine from one mode to the other, prior to this invention, required a complete shutdown of the machine, the removal of one major part of the apparatus (a production wheel), and the installation of another. This is time consuming, and prevents the use of the same tool when different areas of a work piece require different types of particles.
Alternatively, another mechanism such as a grinder has been added after the metering device to grind preformed dry ice pellets in a secondary process step added between metering and conveyance. This method adds to the cost and complexity of the apparatus and often creates plugging and feed backup due to improper management of the near constant need to frequently adjust the grinder for variances in pellet sizes, pellet hardness, desired flow rates and humidity levels. This method also requires pellets to granulate and cannot granulate dry ice in block form.
Until now, most users of dry ice blasting equipment have opted for the more expensive alternative or purchasing two separate machines: one type of apparatus for pellet blasting; and another type of apparatus for granule blasting. It is an object of this invention to provide one apparatus with means in one mechanism to both meter (or otherwise produce) and dispense either preformed dry ice pellets or dry ice granules by the mer reversal of direction of a movable carrier (sometimes called a xe2x80x9cproduction wheelxe2x80x9d). The savings of time and expense are obvious. Only one set of tooling is required, and no part of it needs to be exchanged when particles of different size or nature are desired.
Another advantage of this invention is its adjustability to provide mixtures of different mass sizes of particles.
Apparatus according to this invention is incorporated in a system which includes a supply of pressurized air, a nozzle, a means to convey and inject dry ice particles into a pressurized airstream which passes through a means of acceleration such as a venturi nozzle, and a storage bin to hold dry ice in the form of pellets or block to be metered into the airstream.
A carrier is movably supported and contains two different sets of always-open passages. The carrier is adapted selectively to be driven in a first direction to implement the action of the first set of passages or in a second, different substantially direction to implement the action of the second set of passages. The first set of passages permits passage and metering of dry ice pellets only when the carrier is moved in the first direction. The second set of passages incorporates working edges defining a cutting or scraping surface during movement of the carrier in the second direction and thus simultaneously extracts and meters dry ice granules into the conveyance airstream.
Therefore in one direction of movement or the other, a supply of preformed dry ice pellets of a selected size will be provided to the airlock. To switch from one type of operation to the other, it is necessary only to reverse the direction of direction of the carrier and to provide in the storage bin the appropriate type of dry ice for the type of dry ice particle desired (pellets or granules).
In addition, when pellets are used as a supply, it is possible to adjust the proportion of larger particles and smaller ones to select ratios of particles of various mass (and therefore momentum) and provide a mix of particles.
The above and other features of this invention will be fully understood from the following detailed description and the accompanying drawings, in which: